A large number of materials have been described as optical storage media for analog or digital storage of information with the aid of a laser beam. The information is written into the particular storage medium in the form of holes, pits or bubbles or by phase transformations or other local changes in properties. Storage media which have been described are thin layers of inorganic materials, such as metals, alloys, doped metals, metal oxides or metal sulfides, and of organic compounds, in particular dyes, but also liquid crystal compounds or polymers and combinations of these.
Dyes have a high absorption which can be optimized for the relevant laser wavelength, and are distinguished by low thermal conductivity and a variety of possible methods of processing, (eg. vaporization under reduced pressure, or spincoating with or without a binder or other additives). Their optical data (real and imaginary part of the refractive index [n,k].lambda.) are generally inadequate, so that a high quality memory can be obtained only with matched layer thicknesses or with an additional reflector layer. Moreover, only a few suitable IR dyes are known which absorb sufficiently in the near infrared region (750-900 nm), i.e. in the spectral range in which the technically advantageous semiconductor lasers of the GaAlAs type emit light, and which are used for writing on and reading optical recording media in a reasonable manner. Hence, optimization of such systems requires additives, such as carbon black or oxygen quenchers, which have a synergistic effect but also complicate the preparation of the storage layers.
Although thin metal layers, in particular tellurium and its alloys, and modified materials based on tellurium exhibit suitable optical data (adequate absorption and reflection) in a wide wavelength range, they have relatively high thermal conductivities and are generally very sensitive to corrosion.
It is an object of the present invention to provide a novel optical recording medium, starting from a material which has adequate reflection, is easy to process and has high stability under the conditions of use.
Graphite, which has a layer structure and hence possesses properties of organic and inorganic (metallic) materials, should constitute a suitable storage material.
DE-A-2 150 134 describes an optical recording medium which consists of a thin layer (about 0.5-1.0 .mu.m) of carbon black particles in a polymeric binder, preferably plasticized nitrocellulose, on a heat-resistant substrate (glass). The layer material is selectively vaporized or burned away at the irradiated points by means of a laser beam, and in this way an image is produced.
However, we have found that binder-containing carbon black layers which have been sprayed on have only a low reflectivity owing to the surface roughness; hence, high quality storage media are obtained only with matched layer thicknesses coupled with complete removal of material, and the information density remains restricted (as a result of the large signal area and signal spacing).